1. Field of the Invention
This invention relates generally to an oil pump assembly and, more particularly, to an oil pump assembly that includes a pilot pressure relief valve for equalizing the pump discharge pressure and the control valve pilot pressure.
2. Discussion of the Related Art
Vehicles include an oil pump assembly for pumping engine oil through the vehicle engine to lubricate the engine components through various lubrication channels. FIG. 1 is a cross-sectional view of one type of an oil pump assembly 10 that is used for this purpose in certain vehicles. The assembly 10 includes a metal housing 12 including a pump inlet port 14 connected to an oil source (not shown), a first pump discharge port 16, a second pump discharge port 18, and a pilot pressure inlet port 20. Source oil from the pump inlet 14 is drawn to a first stage pump 24 and a second stage pump 26. The inlet port 14 is also the low pressure exhaust for the pumps 24 and 26. The pumps 24 and 26 are rotated by a chain sprocket mounted on the front of a shaft 28 and can be any suitable pump gear configuration, such as a gerotor assembly. Operation of the pump 24 pumps oil to the first discharge port and operation of the pump 26 pumps oil to the second discharge port 18. The first discharge port 16 is always in fluid communication with the engine 17. The first and second discharge ports 16 and 18 are sometimes in fluid communication based on the position of the control valve spool. The pilot pressure applied to the pilot pressure inlet port 20 is the pressure of the oil in the engine's lubrication channels.
The pump assembly 10 includes the two pumps 24 and 26 to provide the pump discharge flow at both the discharge ports 16 and 18 so that the total discharge flow can be controlled. As the vehicle speed changes the pump speed changes, and thus the discharge flow changes. Therefore, it is necessary to regulate the discharge pressure during vehicle operation to satisfy the engine's oil pressure limits. To provide this regulation, the pump assembly 10, with its control valve, selectively adds or removes the second discharge flow at the discharge port 18 and regulates the first discharge flow at the first discharge port 16. The valve exhaust flows are routed back to the oil source through the pump inlet port 14. The degree and rate at which the first and second discharge flows are changed is determined by the pressure at the pilot pressure port 20.
A valve spool 32 positioned within a housing chamber 34 is employed to change the pump discharge flow when the pilot pressure changes during vehicle operation. The chambers 34 are in fluid communication with the first and second discharge ports 16 and 18 and the pump inlet port 14. A valve spring 36 is positioned within a chamber 37 and a spring land 38 of the valve spool 32 opposite the pilot pressure inlet port 20, and biases the valve spool 32 towards the pilot pressure inlet port 20. The valve spool 32 further includes an intermediate land 40 and a pilot pressure land 42. A narrowed section 44 connects the spring land 38 to the intermediate land 40, and a narrowed section 46 connects the intermediate land 40 to the pilot pressure land 42 to give the valve spool 32 its general shape as shown.
When the pilot pressure at port 20 is below a certain pressure, the valve spool 32 is in the position shown, and the discharge ports 16 and 18 are in fluid communication to allow the complete discharge flow from both the pumps 24 and 26 to lubricate the engine. Also, the discharge ports 16 and 18 are blocked to the exhaust 14. When the pilot pressure increases above a certain pressure, it pushes against the pilot pressure land 42 through the port 20, and the valve spool 32 moves against the bias of the spring 36. Movement of the land 40 begins closing off the second discharge port 18 from the discharge port 16 to reduce the oil discharge flow and pressure, and also begins opening the discharge port 18 to exhaust through channel 48. As the pilot pressure continues to increase, the land 40 will eventually completely close the second discharge port 18 so it is not in fluid communication with the first discharge port 16, and completely open the second discharge port 18 to the exhaust.
If the pilot pressure continues to increase, the valve spool 32 will move far enough against the bias of the spring 36 to cause the spring land 38 to allow the first discharge port 16 to gradually be opened to exhaust through channel 50. Eventually, the spring land 38 will contact the chamber wall 52 preventing further movement of the valve spool 32 in that direction. If the pilot pressure decreases, the valve spool 32 reverses its direction as just described to increase the oil discharge pressure. Thus, the valve spool 32 allows regulation of the engine oil pressure. Operation of the pump assembly as just described is well known in the art.
A problem exists with the operation of the oil pump assembly 10 described above that causes it to be unstable. FIG. 2 is a graph with time on the horizontal and pressure on the vertical axis showing a discharge pressure line 54 for the combined first and second discharge pressures and a pilot pressure line 56 for the pilot pressure. As the discharge pressure rises, the pilot pressure also rises with a certain time lag depending on the particular system, because the pilot pressure is taken from a point inside the engine and not the pump discharge in this design. The time lag as being described herein is directly related to the fact that the pilot pressure is the pressure within the lubrication system and not the actual discharge pressure. This lag can be eliminated by sensing the discharge pressure as it leaves the assembly 10, but certain lubrication system designs benefit from measuring the pilot pressure at this location, as is known in the art.
Once the pilot pressure increases to a certain level, the actuation of the valve spool 32, as discussed above, causes the discharge pressure to begin to decrease at time 58. Because of the lag between the change in the discharge pressure and the associated change in the pilot pressure, the pilot pressure does not immediately decrease when the discharge pressure starts decreasing. Once a certain period of time passes, the pilot pressure begins to decrease at time 60. But, the discharge pressure has been continually decreasing for the period of the time lag. Eventually, the discharge pressure will begin to increase at time 62 because the pilot pressure has been decreasing for some time, but the pilot pressure is still decreasing because of the lag. This process continues through the operation of the assembly 10. This instability in the operation of the pump assembly 10 causes pressure oscillations and added wear and tear on the pump and engine components, reducing device longevity.
It would be desirable to eliminate the lag of the pilot pressure in the oil pump assembly 10 to decrease the oscillation of the valve spool 32 to allow the pump 10 to operate more smoothly. It is therefore an object of the present invention to provide an improved valve spool for the oil pump assembly 10.